A gas turbine engine has a compression section, a combustion section, and a turbine section. The compression and turbine sections have at least one rotor stage. Each rotor stage includes a disk which rotates about the axis of the engine, and a circumferential row of rotor blades extending radially outwardly from the disk into a flow path of working medium gases. Each blade has a platform which provides a boundary to the flow path. Radially inward of the platform is a blade root which engages a blade retaining slot in the disk. In some rotor designs, the slot extends circumferentially about the rim of the disk.
The platforms of adjacent blades are circumferentially spaced from each other, and working medium gases can leak from the flow path, through the gap between adjacent platforms, and then through the blade retaining slot. Also, the platforms are radially spaced from the disk rim, and the gases can leak under each platform, and through the blade slot. Such leakage of gases, from a region of high pressure to a region of low pressure, is undesireable, as it decreases the operating efficiency of the engine.
Examples of seals for limiting the leakage of gases through a circumferential blade retaining slot of a rotor disk are shown in U.S. Pat. Nos. 3,972,645 and 4,464,096. Both patents describe annular ladder seals, having circumferentially spaced apart crossbars connected at opposite ends by circumferentially extending strips, the crossbars and strips forming a ladder shaped member. The blade retaining slot in these patents includes a circumferential recess having opposed, axially facing sidewalls, and the seal is located within the recess and beneath the blade platforms. Each blade root extends through an opening between adjacent crossbars and engages the slot. In U.S. Pat. No. 3,972,645, the axial width of the seal is equal to the axial width of each blade platform, and each of the crossbars is in overlapping relation to the gap between adjacent blade platforms. During engine operation, centrifugal forces cause the seal to move radially outwardly into contact with the underside of the platforms to seal the gap and limit interplatform leakage of gases. In U.S. Pat. No. 4,464,096, the ladder seal has crossbars which, in the as-fabricated condition, are bowed radially inwardly. When the engine is at rest, the axial width of the seal is less than the distance between the recess sidewalls. During engine operation, the seal is forced radially outwardly into contact with the underside of the blade platforms. This causes the bowed crossbars to be flattened against the platforms, and the seal spreads apart in the axial direction until the circumferential ends of the seal contact the axially facing recess sidewalls. Such contact limits the leakage of gases beneath the platforms and through the blade slot. The as-fabricated axial dimension of the seal, as well as the degree of crossbar bowing, must be precisely controlled to insure that during engine operation, the seal ends will contact their respective sidewalls.
In both of the above referenced patents, if the seal fractures during engine operation, pieces of the seal may enter the gas stream and cause foreign object damage to the engine components. In order to limit the escape of such pieces, radial and axial clearances between the blade platforms and the blade retaining slot in the disk must be minimized, which may complicate machining and assembly of the rotor.
Other patents which indicate the general state of the art in the field of blade root seals are U.S. Pat. Nos. 1,003,892, 1,276,405, 2,299,429, 3,266,771, 3,367,624, 3,700,354, 3,972,645, 4,029,436, 4,101,245, 4,183,720, and 4,455,122.